The present disclosure relates generally to communications networks, and more particularly, to label retention in a label switching network.
In a Multiprotocol Label Switching (MPLS) network, packets are assigned labels by Label Switching Routers (LSRs) and forwarded along a Label Switched Path (LSP). The label represents a Forwarding Equivalence Class (FEC) to which that packet is assigned. To assure that traffic is forwarded correctly, each LSR creates mappings between FECs and the labels used as an index to a forwarding table at that LSR. A Label Distribution Protocol (LDP) is used by the LSRs to inform other LSRs of the label/FEC mapping it has made for a given FEC, which can then be used by the receiving LSR to forward labeled packets towards the advertising LSR.
Conventional MPLS implementations have two available modes for retaining label information received from a label distribution peer; conservative label retention and liberal label retention. If an LSR supports conservative label retention, the router maintains only label/FEC mappings that will be used for forwarding packets. If an LSR operates in liberal label retention mode, the router keeps all received label/FEC mappings, regardless of whether or not the mapping is from its next hop router for the FEC at a given time.
Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.